In medical laser applications it is often required to combine a laser aiming beam and a laser treatment beam into the viewing path of a microscope to allow a practitioner to view a target area while the area is being treated with the laser beam. It is preferable that the method of combining the laser beams into the viewing path does not degrade the viewing quality or distort the laser beams. An important safety aspect of this arrangement is that the viewer is protected from scattered or reflected treatment beam laser radiation from the target.
Various methods have been used to attempt to achieve these requirements such as that described in United States patent application number 2002/0165525, assigned to Nidek Co Ltd. In this case a mirror is used to direct the laser beams into the viewing path and a separate protection filter is used to protect the viewer from laser radiation. A limitation of this design is that the mirror size and placement must be arranged to avoid interference with the viewing path.
Another arrangement is described in U.S. Pat. No. 5,634,923, assigned to Carl Zeiss Stiftung. The arrangement uses a deflecting optic which has a wavelength selective coating applied to it. The coating is designed so that it will be largely reflective at the treatment laser wavelength(s) used but non-reflective at other wavelengths. This allows the deflecting optic to be placed across the viewing path and has the added advantage that much of the reflected laser radiation is also reflected away from the viewing path. However, a laser protection filter is still required to meet safety requirements and the viewing quality is degraded by the wavelength selective coating which alters the viewing colour balance.
Another arrangement is described in U.S. Pat. No. 5,954,711, assigned to Nidek Co Ltd. In the Nidek arrangement a dichroic mirror is designed to combine the laser beam(s) into the viewing path and perform the function of a laser protection filter. This design also uses a wavelength selective coating, but in addition the substrate material is chosen to absorb any reflected laser radiation which comes into the viewing path. In this way a separate laser protection filter is avoided, however the viewing quality is limited by the change in colour balance in the viewing path.
The problem of altered colour balance is addressed in U.S. Pat. No. 5,966,240, assigned to Coherent Inc. In the Coherent arrangement the wavelength selective coating on the combining optic is designed to give the viewer a more natural colour balance, however a separate laser protection filter is still required.
Each of these combining optic designs using wavelength selective coatings also suffer from astigmatism which lowers the laser beam quality and degrades the viewing quality. This is caused by stress induced curvature of the optic caused by the coating process which introduces astigmatism to the laser beam(s) and also due to the use of angled parallel glass plates as a substrate which introduces astigmatism into the viewing path.
To reduce the stress induced curvature caused by the coating process the coating thickness needs to be kept to a minimum and the substrate thickness increased to resist the stress; however the substrate thickness needs to be kept to a minimum to minimize the astigmatism in the viewing path. These conflicting requirements usually result in a compromise design with a level of astigmatism in the laser beams and the viewing path and a coating which is not completely reflective at the treatment laser wavelength, thereby requiring a separate laser protection filter to be used.
An attempt to correct this astigmatism is described in Japanese Patent number 4164444, assigned to Nidek Co Ltd. Nidek uses a dichroic mirror that is curved. The curvature limits the usable size of the reflective area and a separate laser protection filter is still required.
The problem of astigmatism in laser devices has been addressed in some applications by using a wedge-shaped beam splitter. One typical example is described in U.S. Pat. No. 6,611,383, assigned to LG Electronics Inc. These applications do not have a requirement for a laser safety filter to protect a viewer and do not require thick dielectric coatings.